This invention relates generally to electric power monitoring systems and more specifically concerns correction of phase and magnitude errors introduced by certain components of the monitoring system, in particular the data acquisition components thereof.
The operating efficiency of electric power systems has recently become of greater importance in the electric power industry. Electric power producers and providers are now generally attempting to use their existing power generation plants and transmission lines to a high, if not maximum, capacity, due to consistent increase in customer demand combined with little or no increase in capacity over the past several years.
In order to operate at maximum efficiency, the electric power producers and providers must know how their systems are performing, including the efficiency at which their systems transport power. Power system protective relays are used to monitor efficiency, as they are typically located throughout the power system and have the required ability to measure currents and voltages on the power line and then to determine how the power system is operating.
Monitoring of the power system to optimize operation, and hence revenues, requires high signal acquisition and processing accuracy, within xc2x11/10% (0.1%) or better. Historically, however, protective relays, which have in the past been used for detecting and isolating abnormal, i.e. fault, conditions in the power system could provide satisfactory results for those purposes with accuracies of 2-4%, or even higher.
Errors are typically produced in the data acquisition section of the protective relay. The data acquisition section obtains electric current and/or voltage quantities from the power transmission line on the separate channel inputs and operates on them to produce signals which can then be processed with a protection algorithm. The data acquisition section, however, introduces both phase and magnitude errors into the current and voltage signals obtained from the power line. The components in the data acquisition system responsible for the errors include current and voltage transformers (CTs and VTs), low pass filters (LPFs), analog to digital converters (A/D converters) and multiplexers.
A typical data acquisition section is shown in FIG. 1. It includes a plurality of analog data channels, receiving, for instance, the three phases A, B, and C of a three phase power signal (current and/or voltage quantities). The three phases from the power line on the several channels are applied to a CT or VT element, shown generally at 12, depending upon whether the inputs are currents or voltages, a low pass filter 14, and then to a multiplexer 16, along with signals from other channels. The output from the multiplexer 16 is applied to an analog-to-digital (A/D) converter 18, the output of which is applied to a programmable logic device 20, such as a microprocessor or other similar device, which processes the digitized data by protection algorithms to monitor the operation of the power system.
Briefly in operation, the data (electric current quantities or voltage quantities from the power line) in each analog channel is periodically sampled, under the control of the programmable logic device, applied to a CT or VT as appropriate, then to the low pass filter and from there to the multiplexer. The programmable logic device selects the channels to be sampled in a programmed sequence. The A/D converter 18 converts the analog information from multiplexer 16 to digital data which is then processed by the protection algorithms in the programmable logic device. This process continues until samples from all of the channels have been processed. The cycle is repeated at regular intervals during a power system cycle, e.g. 16 times per cycle or at some other fixed rate.
Ideally, the current/voltage data which is provided by the A/D converter to the programmable logic device is identical or substantially identical in all respects to the current/voltage signal obtained from the power line, at the corresponding point in time of sampling of the analog quantity.
It is well known that errors are introduced into the acquired signal by the individual components in the data acquisition section of the protection system. Further, the introduced errors vary unpredictably from component to component; each receive channel in the relay will thus have different error levels. These errors include both magnitude and phase errors. Some components, such as CTs, introduce both magnitude and phase errors, while other elements, such as the low pass filters, introduce only a phase shift, which is frequency dependent. In addition, the analog multiplexer 16 and the A/D converter can only operate on one channel at a time. Hence, there will be an inherent phase difference between the successive channels for the output of the A/D converter.
Various attempts have been made to compensate for the magnitude errors and phase delays in the data acquisition section of the protective relay. One method of magnitude calibration is to apply a known signal to the CT/VT inputs and compare them to the digital value at the output of the A/D converter. The difference is then calculated as a gain factor, and this gain factor is used to correct the output of the A/D converter, prior to the application of the digital signal as an input to the programmable logic device.
For phase shift corrections, adjustments can be made to the individual hardware components to correct phase error. However, this is expensive and labor intensive.
Correction to differences in phase between the channels due to sequential sampling may be accomplished by various techniques, including rotation of sampling points or reversal of sampling sequence, but these do not correct the phase shift introduced by the action of the individual components themselves. It is thus desirable, in order to accomplish the accuracy required in the monitoring of modern power systems, to reduce both magnitude and phase errors down to a very small percentage and to account for all sources of such errors, including a change in error as the magnitude of the input signal changes.
Accordingly, the present invention is a system and corresponding method for compensating for errors introduced into electric power monitoring systems by a data acquisition section thereof, which in operation is responsive to analog quantities from an electric power line to produce sampled digital signals which are used in a monitoring algorithm, the system comprising: a test unit for applying a sinusoidal test input signal to a front end portion of the data acquisition section; a processing module to which the test input signal is also applied for converting the test signal to a module output signal having falling and rising edges corresponding to transitions between positive and negative portions of the sinusoidal test signal; means for determining a zero crossing of the sampled digital signal and for determining the time difference between an edge of the module output signal and said zero crossing of the sampled digital signal, wherein said time difference is representative of the phase shift produced by the data acquisition section; and a compensation circuit for phase adjusting the sampled digital signals in accordance with said time difference.